Most fabric that is dyed, printed, or finished must first be prepared, with the
exception of denim and certain knit styles. Preparation, also known as
pretreatment, consists of a series of various treatment and rinsing steps critical
to obtaining good results in subsequent textile finishing processes. . In
preparation, the mill removes natural impurities or processing chemicals that
interfere with dyeing, printing, and finishing. Typical preparation treatments
include desizing, scouring, and bleaching. Preparation steps can also include
processes, such as singeing and mercerizing, designed to chemically or
physically alter the fabric. For instance, the mercerizing stage chemically
treats the fabric to increase fiber strength and dye affinity, or ability to pick up
dyes. This, in turn, increases the longevity of fabric finishes applied during
finishing. Many of the pollutants from preparation result from the removal of
previously applied processing chemicals and agricultural residues. These
chemical residues can be passed on to subsequent stages with improper
preparation.
Most mills can use the same preparation equipment for the entire range of
products they produce. In most cases, facilities favor continuous rather than
batch preparation processes for economic and pollution control reasons. A
number of mills, however, prepare goods, particularly knits, batchwise on dyeing
machines to simplify scheduling and handling. Sometimes, facilities
operate batchwise to reduce high capital costs required for high productivity
and the complexity of storing and tracking goods through continuous wet
processing operations.
Because preparation is relatively uniform across most of a mill's production,
preparation is usually the highest-volume process in a mill and hence an
important area for pollution prevention. If fabrics contained no contamination
upon arrival for wet processing, preparation processes would be unnecessary,
eliminating about half the pollution outputs from wet processing and a
significant amount of wastewater. The primary pollutants from preparation
is wastewater containing alkalinity, BOD, COD, and relatively small amounts
of other contaminants such as metals and surfactants.
Following are major fabric preparation techniques
- Singeing
- Desizing
- Scouring
- Bleaching
- Mercerizing